Telephone transmitter



Sept. 2, 1924. 4 1,507,081

B. F. MIESSNER TELEPHONE {TRANSMITTER Filed llarc h 12. 1919 3 Shoets-Sheot' IHIIIIIIUIIIIIIIIIIHI A ATTORNEY Sept. 2; 1924., 1,507,081

. B. F. MIESSNER TELEPHONE TRANSMITTER Filed March 12 1919 3 Sheets-Sheet 3 4 ATTORNEY.

Sept. 2, 1924.v 1,507,081

4 B. F. MIESSNER TELEPHONE TRANSMITT s R Filed March 12 1919 3 Sheets-Sheet 5 a II 1111111111111 WITNESSES:

. Patented Sept. 2, 1924.

UNITED STATES PATENT OFFICE.

BENJAMIN I. MIESSNER, NEW YORK, N. Y., ASSIGNOR, BY MESNE ASSIGNMENTS,

TO THE MAGNAVOX COMPANY, OF OF ARIZONA.

SAN FRANCISCO, CALIFORNIA, A. CORPORATION TELEPHONE TRANSMITTER.

Application filed larch 12, 1919. Serial No. 282,155.

To all whom it may concern.

Be it known that I, BENJAMIN F. Mmss- NER, a citizen of the United States, residing at New York city, in the county of New York and State of New York, have invented certain new and useful Improvements in Telephone Transmitters, fully described and represented in the following specification and the accompanying drawings, forming a part of the same.

The invention was suggested by the necessity for telephoning in or from airplanes, where the noise of the engines interferes seriously with the operation of ordinary telephone transmitters, and by observation of the entire or partial failure of methods and devices intended to overcome this difiiculty.

The invention therefore comprises a method and means for substantially eliminating objectionable noises which are present in an airplane or other noisy places, while properly transmitting desired sounds, usually the human voice.

Stated in more general-terms, the object of the invention is to accomplish the clear and satisfactory telephonic transmission of certain sounds with the substantial elimination of other sounds.

In general terms the method of accomplishing this object consists in causing the sound-responsive element, such as a diaphragm, to respond to sound vibrations from a source relatively near and substantially opposite one face of the diaphragm, and causing it to be substantially unresponsive to sound vibrations proceeding from sources relatively distant, wherever located, or those from a source relatively nearly in line with the plane of the diaphragm, whethor near or distant from the diaphragm.

More particularly stated, the method consists in causing or permitting sound vibrations which it is desired to transmit, and which may therefore be identified as desired sounds, to impress themselves effectively on one side of the diaphragm so that the resulting vibrations of the diaphragm may be converted into electrical vlbrations for transmission, while causing or permitting sound vibrations which are not desired for transmission, and may be identified as undesired sounds, to exert equalizing or neutralizing effects on the two sides of the diaphragm so that the diaphragm is in effect substantially inert or unresponsive to such s0un $till more particularly the method consists in permitting equalizing access to the diaphragm of sounds of the second class from any relatively distant source and any direction, and in some cases positioning the plane of the diaphragm to produce or increase an equalizing effect with respect to sounds proceeding from certain sources, whether near or distant.

In general terms, the apparatus for accomplishing the desired object consists in a diaphragm exposed on both sides to contact by air pressure variations caused by sound vibrations and substantially balanced with respect to undesired sounds, while unbalanced with respect to desired sounds, and also substantially balanced with respect to sounds proceeding from directions approximately in line with its plane, without regard to their classification as desired or undesired sound; and suitable co-operating means for converting the vibrations of the diaphragm into electrical vibrations for transmission.

The operative principle of my invention is such, as more particularly explained hereinafter, that when sounds of diflerent characters proceed from a point located at any given distance from a face of the diaphragm, such distance being substantially greater than the distance at which a sound source is to be placed with the purpose of reproduction, the extent to which the diaphragm is unaffected (or in other words, the de ree to which elimination of such sounds is e ected) depends upon the rate of pressure variation in the diiferent sound waves; and factors which determine this rate include pitch, amplitude, and specific Wave form.

The accompanying drawings comprise certain diagrams to graphically explain the theory involved and also show certain forms of apparatus embodying the invention. The method forming a part of the invention is carried out in connection with these or other forms of apparatus. After studying the drawings in connection with this specification, persons skilled in the art will understand that the method and the apparatus are subject to reasonable variations within the spirit of the invention, and I do not limit myself to details except as claimed. In the drawin s:

Fig. 1 is a view showing aiconventionalized way a telephone transmitter of common form in a certain position in relation to sound waves also represented conventlonallg 1g. 2 shows the same transmitter in another position;

Fi .3 is a conventionalized showing of a nove diaphragm arrangement;

Fig. 4 is a diagram, showing the device of Fi 3 in a diiferent relation to sound waves.

. Fig. 4 shows the same device in another arrangement in relation to sound waves;

Fig. 5 is a diagram illustrating the efiect of a sound wave of a certain sort on a certain diaphragm;

Fi 6 is a'diagram showing the efiect of a di erent sort of sound;

Fig. 7 is a diagram showing the action of still another sort of sound;

Fig. 8 is a diagram illustrating the effect of sound shadow;

Fig. 9 is a sectional view of a typical simple magneto transmitter embodying my invention;

Fig. 10 is a view with certain parts in section and certain parts in elevation of a transmitter of the carbon button type embodying my invention in one form,

Fig. 11 is a rear elevation of the same with the rear casing removed;

Fi 12 is a section of the structure of Fig. 10 with the rear casing removed;

Fig. 13 is a sectional view of a simplified carbon transmitter embodying my invention; and

Fig. 14 is a front elevation of the same;

Fig. 15 is a vertical section of a modified form of transmitter.

16 is a transverse section of the same.

The development of a method and apparatus for producing the desired result followed a theoretical study of the nature of sound vibrations in relation to means for telephonic reproduction of them, and orderly experiments directed by such theoretical stud A clear understanding of the inventive problem and the solution for it provided by the invention will be facilitated by a brief theoretical discussion based upon thestudy referred to:

Under ideal conditions sound proceeds from a source in the form of an enlarging sphere in which at any point the sound acts upon a responsive medium by reason of variations or oscillations in air pressure, and

.it is these ressure variations, which might be describe as definite pressure impacts (in some cases negative as well as positive), which cause the movement of the responsive medium, such as a tele hone transmitter diaphragm. The wave ont form defined by a circle whose center is the sound source, will evidently have a difierentcurvature in comparison with any definite datum at differeiit distances from the source, so that for descriptive purpgses the wave front form may be said to relatively sharply curved near the source and relatively straight or flat at points far removed from the source. Thus, considering a datum dimension of about 3 inches, representing the diameter of a telephone diaphragm, the wave front form of a sound reaching it from a distance of 3 or 4 inches is sharply curved or convex, and the front form of one reaching it from only about 3 feet or a greater distance is evidently practically flat to all intents and purposes.

Sound vibrations difier in their characteristics in ways which can most easily be expressed by graphical representation. Taklng only two examples: a curve representing graphically the cross-section of the ideal sound sphere above mentioned in one of the planes of the source, for a sound of one character will show a sharply peaked form, representing sudden and intense variations in air pressure, while the curve of another sound will show an easy curvature or a substantially sinusoidal form. Differences in characteristics represented by such curves may be referred to a time factor since in the sharply peaked curves the pressure variations of whatever degree are relatively compressed and pass a given point in a brief time while the variatlons represented by the easy curves .are relatively extended and require relatively much more time to pass a given point. Sounds of the same intensity and pitch may have very difierent characteristics represented by peaked curves, or sinusoidalones or almost anything between the two.

In a telephone transmitter as ordinarily arranged (Figs. 1 and 2) a diaphragm d forms substantially the flat side of a closed hemisphere of which the s herical part is the casing 0. Wherever this transmitter is placed with relation to a sound source, air pressure variations are impressed upon all parts of its surface and become efiective upon the flexible diaphragm by bending it in and out as shown graphically in Figs. 1 and 2. The depression of' the diaphragm (Fig. 1) is caused by positive external air pressure, and the bulging or outward curve (Fig. 2) is caused by a negative pressure or one below the pressure of the air within the casing. Figs. 1 and 2 show the diaphragm arranged in dilferent positions with respect to the parallel lines which represent the approximately straight wave front form of sounds coming from a considerable distance, and whose relative proximity to each other indicates the relative intensity of pressure. It is shown graphically in these figures that the diaphragm responds to the pressure variations regardless whether its plane is parallel to the sound 'wave front or in any other position. Such a diaphragm will respond to the sound of the voice roceeding from a point opposite to it and ro m a distance of only 2 or 3 inches, and 1t will also respond to any sound of suflicient intensity proceeding from any direction and from any distance. There is no selective capacity and any considerable extraneous sounds are transmitted along with the desired sound which is usual y that of the speaking voice.

Various means for excluding undeslred sounds have been proposed in order make it possible to use a te ephone transmitter 1n noisy places. These devices have failed partially or entirely to give the desired results for reasons understood by'persons skilled in f this art.

My theoretical study above referred to, and study of the practical dlfiiculties 1 nvolved in telephonic transmission under d fficult conditions, as for example, 1n an airplane in proximity to very noisy Internal combustion engines, led me to experimentwith a diaphragm supported in a relatively thin ring 1' shown graphically 1n Fig. 3, and open to free access of air pressure variations on both sides. a diaphragm would vibrate only when the influencing sound waves possessed certain characteristics of direction, curvature, frequency and wave form, and that it Wlll not respond to vibrations advancing 1n the direction of its plane, as shown in Fig. 3, because as clearly indicated there, equal and oppositely directed pressures are producad on its two sides. Whenwaves advance in directions approximately perpendicular to the plane of the diaphragm, it is found that their effect depends chiefly upon the wavefront form and other characteristics of the waves and the thickness of the sup orting ring; or, otherwise stated, the effect epends upon distance of the source and the rate of wave-pressure variation, with due regard to ring-thickness.

Sounds desired for reproduction, to which the diaphragm must therefore respond accuratel are usually those of the human voice,and will usually proceed from the lips, held opposite and within two or three inches of the diaphragm, or even closer, as indicated in Fig. 4, in which 8 represents a sound source located as above, and the curved lines concentric with the source represent the relatively sharply-curved front-form of the sound waves, any given pressure zone of which, as is obvious from this diagram, strikes the front of the dia hragm and produces the corresponding iaphragm movement, before the same pressure zone can reach, and affect, the rear surface. A concurrent cause of diaphragm response, and prevention of equalization by such sounds as are now in consideration, is indicated diagrammatically in Fig. 8, which shows F It was discovered that such f part of the sound waves are reaway from the dia hragm that a large flected back without opportunity to reach and a cot the .fileiragm decreases; that is, the wave front comes flatter in relation to any given linear dimension, such as the diameter of the diaphragm. When the sound source is three eet or more from a diaphragm of about three inches in diameter, the wave front form at the diaphragm is, to all intents and purposes, flat, as indicated in Figure 4. This flattening of the wave front reduces the responsiveness of the diaphragm to the sound vibrations, and conversely produces an equalizing effect upon the two sides of the diaphragm when the ring 0" is of suitable thinness, until at a certain distance which may be only two, three or four feet or a great variety of sounds of moderate intensity, the diaphragm is practically unresponsive to their vibrations. This equal-- izing action due to the flattening of the wave front can best be understood by considering an ideal representation of the cross-sectional form of a certain sound wave, as shown in Fig. 6. Here the elevation of different points of the curve a above the base line 7; indicate the degree of pressure at that point of the curve. When the ring 1' is of moderate thickness t in relation to the diameter of the diaphragm d, it is apparent that with a substantially flat wave front the pressure upon the front face of the diaphragm at any instant is substantially balanced by an almost identical pressure on the back of the diaphragm. This equalization is also graphical y indicated in Fig. 4, in which the parallel lines represent the wave front form, which is substantially straight, as shown, when the sound source is considerably distant from the diaphragm, and the relative proximity of these lines indicates the degree of pressure. This equalization is not primarily dependent upon reduction in loudness of the sound as the source is moved farther from the diaphragm, but upon the relative straightening of the wave front, as

above explained. Thus, sounds received from sources two or three feet from a face of the diaphragm will fail to vibrate the diaphragm appreciably in many cases when a sound of otherwise similar character proceeding from a source much nearer the diaphragm, say two or three inches, and no ouder at that point than the other sound, will cause ample vibration, as indicated in When a diaphragm supported and arranged as above described is employed with suitable means for converting its vibrations into electrical impulses, as shown and described hereafter, 1n certain telephone transmitters embodyin the invention, the voice of a person speaking into the transmitter in the normal way is correctly transmitted for reproduction, and a great variety of other sounds, if they are being produced in the vicinity, and which would ordinarily affect the transmitter and interfere with the clear transmission of the desired sounds, are eliminated, for the reasons above explained. For example, if these undesired sounds are the voices of other persons talking in the vicinity, even when such persons are located substantially opposite the diaphragm of the instrument at either front or back, and only two or three feet distant, the sound of their voices is largely or entirely eliminated. Many other sounds from similarly located sources are also eliminated. When sounds have certain characteristics, as later explained, which make their elimination es ecially difficult, and when these difiiculties are so great that the phenomenon of flattening of their wave front form in relation to the diaphragm is not suiiicient to produce an equalizing effect upon the diaphragm to eliminate such sounds, complete elimination may be accomplished by turning the instrument so that the plane of the diaphragm is more or less in line with the sound source, as previously explained.

The completeness of elimination of any sound, with its source at any given distance more than, say, three feet, from a face of the diaphragm, depends upon various characteristics of that sound, such as pitch, amplitude or loudness, and other factors, as sufliciently ex lained in greater detail hereafter, and a or considering such detailed explanation of difi'erent characteristics and their effect upon diaphragm structures of my invention, it will be found that the degree to which elimination of different sounds, at any given distance from a face of the diaphragm, is effected, depends upon the rate of pressure variation in the different sound waves.

A sound wave having a very pronounced peak formation, when proceeding from a source opposite to the diaphragm, and even when the source is a considerable distance away from .the diaphragm, say five or six feet, may produce a sudden pressure on the front of the diaphragm before its speed of advancement has carried it to the other side and effected neutralization; and this may happen in spite of the fact that the wave front form in such a case is substantially flat, which would be suificient to cause neutralization in the case of a sound of less intense pressure variation. This ma occur, for instance, when the wave lengt is comparatively short in proportion to the thickness of the ring. This is shown gra hically in Fig. 5 where w is the curve indicating variation in pressure due to the sound vibration, b is the base line of neutral or atmospheric ressure, Z is one-half of a wave length, amfp is a point in the pressure curve parallel with the front of the diaphragm. The sound wave is su posed to strike the diaphra in substantially at the center so that in t e selected example the pressure point p is sup osed to be applied to the center of the iaphragm, but for greater convenience in applying reference characters the curve is shown at one side of the diaphragm. The vertically arranged numerical ordinates indicate comparative pressures both positive and negative. The point p designates a pressure point in the curve which has passed the diaphragm.

The dimension t indicates the thickness of the ring which is small in relation to the diameter of the diaphra m. The dimension 2 indicates a distance 0 travel of the sound wave equal to twice the thickness of the ring, and this is the horizontal dimension between the front of the diaphragm and the point p of the curve. Evidently the point 12 before its effective pressure can be exerted upon the back of the diaphragm must have travelled a distance equal to t in the direction of the arrow and an equal distance back again before it can become effective on the back of the diaphragm. The not effective pressure upon the face of the diaphragm at the moment indicated in the diagram is therefore the difference between the pressure at p and that at p which, by consulting the numerical ordinates, is seen to be 8; that is, a positive sharp pressure impulse of an intensity equal to 8 in a total positive ressure range of 10 is impressed upon the iaphragm. The foregoing makes it clear that the thickness of the ring has an important effect upon response of the diaphragm to sounds represented by a curve of the sort shown in Fig. 5, for evidently if the ring is thicker the part of the curve to the rear of the diaphragm may be of a negative pressure value, for example, and in this case, the pressure effect upon the face of the diaphragm will be much greater than the maximum sound value properly attributable to the sound curve in question. Also taking the same sort of curve or one which is even more compressed or with more sharply accentuated pressure peaks, it can easily be understood that with a rin of increased thickness which dimension is not in conformity with the wave length of the sound in question, it is readily possible for the pressure variations impressed upon the two sides of the diaphragm to be thrown entirely out of phase so that an entirely false vibration will be set up in the diaphragm.

It is therefore evident that, to eliminate sounds of the character just described, the ring 9- should be made as thin as is practicable. When the ring is sodesigned, substantial equalization is effected, according to the principles above explained, even by sounds having sharply-peaked curves described, within ranges of pitch and intensity usually encountered, when the sounds proceed from sources approximately opposite, and sufiiciently distant from the diaphragm so that the wave front form at the diaphragm is substantially flat.

A sound vibration of the character represented by the curve w, Fi 5, ma be one which it is desired to repro uce, an in that case when the sound proceeds from a source such as the mouth of a speaker held opposite the diaphragm and near it, the sound will evidently be responded to correctly by the diaphragm because of the sharply curved front formation of the sound waves.

when proceeding from a distance which, in the assumed case, is usually no greater than the diameter of the diaphragm and because of the concurrent sound shadow effect produced behind the diaphragm with respect to sounds proceeding from a point close to its front, as graphically shown in Fig. 8.

But the same curve may also be representative of an undesired sound proceeding from a greater distance, and from a oint more or less nearly opposite a face 0 the diaphragm. In that case. the flattening of the wave front caused "b removing the sound source to a greater istance, tends to cause equalization, and the distance at which equalization will be efiected for a sound of the characteristics described depends to a certain extent upon the sharpness of the sound curve, or in other words, upon the rate of sound pressure variation. With certain sounds having a short wave length with rapid pressure variation, equalization is facilitated. as explained in connection with Fig. 5, by making the ring 7' as thin as reasonably possible. This emphasizes the importance of the time factor, or time-and-distance factor briefly mentioned above, since sounds of the character mentioned even when proceeding from a considerable distance and from a point nearly opposite the diaphragm would cause it to respond if the ring were of sufficientthiokness to prevent pressure exerted upon one side from being approximately equalized by the same pressure value reaching the other side without appreciable delay.

When a sound'has very rapid pressure variations, making its elimination correspondingly difficult, although the response of the diaphragm to such sound is proportionately reduced as the diaphragm is removed from the source (while maintaining the plane of the diahragm approximately perpendicular to t 0 direction of the source), a sub stanti-ally complete elimination of such a sound might not be possible without removmg the instrument to an impracticably great distance. In such cases, responsiveness of the instrument to the sound may be entirely eliminated, even with the instrument in close proximity to the sound source, by positloning it so that the plane of the diaphragm is approximately in line with the direction of the source, as previously explained.

If the wave curve is of less pronounced form, such as a sinusoidal form shown in Fig. 6, where the wave is long in proportion to the thickness of the ring 7-, it will be evident that as the wave advances upon the diaphragm it will not produce any appreciable response because'the pressures upon both sides of the diaphragm become practically equalized with unappreciable delay. In other words, such a sound has a capacity to relatively easily efi'ect equalization of a diaphragm arranged according to my invention, and equalization of the diaphragm in respect to such sounds occurs when the diaphragm is removed only a relatively short distance from their source, even when the source is directly opposite a face of the diaphragm.

A sound represented by such a curve may be one of the voice sounds desired for transmission, proceeding from a source substantially opposite and near "the diaphragm. In that case equalization is undesirable, and is prevented by the sharply curved wave front formation due to the proximity of the sound source, or otherwise explained by the sound shadow efl'ect exemplified in Fig. 8.

Another case requiring consideration is one in which a sound is of a character represented by a curve having a relatively short wave length and a smooth or sinusoidal curve as shown in Fig. 7. It would theoretically be possible to secure a pressure impulse greater than the maximum pressure in the wave, unless ring 1* is quite thin. In this case the negative pressure at 7)" added to the positive pressure at p is evidently considerably greater than the true maximum positive or negative wave pressure of the curve indicated by P. Therefore, in this case, with a relatively thick ring the sound proceeding from a considerable distance and having a fairly flat formation would in some cases not equalize the pressures upon the diaphragm properly for eliminating the sound. In an extreme case of this sort it is possible for the maximum negative ressure effect to 'be applied to one side 0 the diaphragm at the moment when the maximum positive pressure is applied to the other side. This would, when converted into an electrical vibration and reproduced, have the effect of substantially doubling the true intensity of the sound. In the case of voice sounds this and similar phenomena would responds to a wave length of about 2 inches.

If a wave be ofsinusoidal form the half wave or pressure zone would be 1 inch long. If the ring 9* were 1} inch thick, 2 in Fig. 7 would equal 1} inch or i of the wavelength. The case Just assumed re resents, therefore, probably the worst condition that is to be met with (considering ideal or simple, wave forms), and to meet this difiiculty without positioning the diaphragm angularly with relation to the undesired sound in question, the ring should be made thinner, and in this case the difliculty would be avoided.

If a given supporting ring cannot be made thin enough to entirely eliminate the difiicult sound conditions such as just described, within structural limitations, the difliculty may be met in that case partially or entirely by angular positioning of the diaphragm.

With regard to all characters of sounds proceeding from any distance from 2 or 3 inches up to many feet (with reference to a diaphragm of about a three inch diameter) and from points more or less nearly in line with the plane of the diaphragm, equalization is effected as shown graphically in Fig. 3, by the fact that the pressure variations of all such sounds reaching the diaphragm from such directions are equalized on the two sides of the diaphragm with sufiicient promptness by permitting the vibrations to reach both sides of the diaphragm with substantial freedom.

The discussion of sound sources and the movement and character of sound waves has been based on simple and ideal forms, and the curves given represent such ideals forms. Actually thequestion is complicated by the inherently complex nature of most sounds, and by sound reflections, and other phenomena. But I am convinced by my study that all complications may be reduced for the purposes of this invention to a combination of the simple problems enunciated, and I find that the method and apparatus here disclosed will act under all ordinary conditions to substantially solve the problem propounded, for practical purposes.

Fig. 9 shows a simple magneto transmitter modified and adapted to embody my invention. The diaphra m D is held in a relatively thin ring R which may of course for convenience in assembling consist of separate parts, namely, a rear member 1 and a front member 2 secured together in any convenient way. Attached to the rear part 1 of the ring is a frame or s ider consisting of two or more arms 3 w ich' carry.

D may be entirely ,.unobstructed except or the very slight'marginal overlap of the ring member 2, and the rear face is substantially unobstructed since free access of air over a large area of the rear face is afiorded by the skeleton or open construction of the frame 3. The freedom of access of vibrations to the rear of the diaphragm may be increased when it is considered desirable by makin the magnet-windings 6 relatively short an placing them toward the rear so that the pole-pieces 6 extend considerably from the front of the magnet, windings. The relatively insignificant area of these pole-pieces adjacent to the diaphragm does not obstruct the latter in respect to sound vibrations to any. appreciable extent. Otherwise, when a certain amount of obstruction of the rear of the diaphragm is caused by the presence of magnets or other parts, this effect may be counterbalanced or neutralized if desired by providing a shield or screen 2 which in Fig. 9 is in effect an inward extension of the outer flange of the ring 2, and this may have a central aperture 2 designed to more or less closely equal the exposed rear area of the diaphragm for balancing effect. In this way air pressure variations caused by sound vibrations are permitted to reach both sides of the diaphragm with suflicient freedom to produce substantial equalization and to eliminate undesired sounds, while desired sounds impressed upon the front of the diaphragm from a point a few inches distant would be properly responded to by the diaphragm without equalization, these'effects causing 'the diaphragm constructed and arranged as shown in Fig. 9 to respond to vibrations of the different characters substantially as previously explained in connection with the conventionally shown diaphragm of Figs. 3 to 8. The vibratory responses of the diaphragm to the desired sounds are converted into electrical impulses for transmission by magnetic induction in the usual way, and since undesired sounds are not responded to by the diaphragm for the reasons explained, they will not be conve'ted into electrical impulses or transmitte Q The method constituting one principal ton transmitter modified and adapted to em-,

body my invention. The diaphragm D is carried in an insulating pac ing 7. This in turn is carried in a recess 8 in ring 9 which is quite thin in relation to the diameter of the diaphragm. A frame or yoke l0 is screwed to the back of ring 9. Damp- I ing devices comprising springs 14 are secured to the frame 10, or to the ring 9, and

exert pressureagainst the rear face of the diaphragm through "insulating pads 15. The frame 10 supports the variable carbon resistance device, which comprises as usual a cup-like receptacle 16, a back disk 17, a granular filling 18, and a button 19, the reduced central portion of which passes through a mica dia hragm 20. The button 19 is connected to diaphragm D by a stem 21 and suitable securing devices. Binding posts 22 and other electrical connections are provided as usual.

Mainly for *mechanical protection of the parts and for convenience in supporting the instrumenta rear casing or housing 25 is provided. This is secured in any convenient way to the ring 9 and may preferably be provided with a lug or car 26 bored for connection to a. suitable standard. The casing 25 is provided with a multiplicity of holes 27 to permit sufliciently free access of air pressure variations to the interior of the casing and so to the back of the diaphragm.

The front of the diaphragm might be entirely exposed except for the slight overlap of flange 28 of ring 9, but in an arrangement preferred in some cases, as in Fig. 10, this flange may be continued inward to provide a covering or screen 29 for the front of the diaphragm. Such front covering or screen, however provided, will be formed to permit the necessary free access of sound Vibrations to the diaphragm, as by being provided with a multiplicity of holes 30. The number and arrangement and total area of the apertures in the front cover and the rear casing when these are employed may be proportioned to each other with sufficient accuracy to permit sufliciently free access of sound vibrations to both sides of the diaphragm to bring about the desired unresponsiveness of the diaphragm to certain sound vibrations, that is. those proceeding from a source relatively distant from the diaphragm or which are in the plane of the diaphragm, while leaving it fully responsive to vibrations from a source relatively near and substantially opposite the face of the diaphragm, such as words spoken to the transmitter in the usual way; this for the reasons previously explained.

Evidently in some cases the rear casing may be omitted and a support for the instrument may be provided at the central member of frame 10 or otherwise, and the front cover or screen 29 may also be omitted when desirable.

A carbon button transmitter comprising only the parts described above may in some cases respond to sound'vibrations having access to the rear of the diaphragm sufliciently to cause more or less disturbance. That is, such vibrations of the button itself will transmit sounds additional to those impressed upon the diaphra m or will neutralize the action of the diap ragm to some extent. To provide against this difficulty I provide means for insulating the carbon button from sound vibrations so that it is responsive only to the mechanical vibrations transmitted to it from the diaphragm. Fig. 12 shows one arrangement for this purpose in which a cylindrical housing 31 is screwed upon the front end of cup 16 forming a continuation of the carbon resistance supporting device. This housing contains an internal flange 32 provided with a central aperture fitting the stem 21 very closely but yet with suflicient clearance to avoid interfering with the reciprocations of the stem.

This flange substantially prevents passage of air. to and from the chamber within it so that air pressure variations are substantially prevented from having access to the button. Other means for excluding air and air pressure variations may also be provided, as for example, a rubber disk 33, cemented or otherwise secured to the front face of housing 31 and secured at its center to the stem 21 by suitable holding devices.

Figs. 13 and 14 show a carbon transmitter of simpler'form modified and adapted to embody my invention. The frame cornprises an annulus 35 against the front edge of which the diaphragm D is clamped by a flanged ring 37, having spring-fingers 3G engaging annulus 35. The frame also comprises a center-piece 38 to which insulating pieces 39 and a carbon disk 40 are secured in any usual or convenient way. The frame also comprises two or a small number of arms 41 connecting the center-piece with the annulus 36. These arms provide very wide and free spaces through which air pressure variations may reach the back of the diaphragmw. The ring 37 may somewhat overlap the front edge of the diaphragm in order to hold it in position, but is usually provided with a large central opening 42, or in some cases may be provided with a center-piece 43 connected to the outer portion of the ring by arms 44, and the center-piece may be provided with perforations 45. The diaphragm D as shown lies close to the disk 40 and bears against carbon pellets or granules 46 in the recesses 47 of the disk as usual in transmitters of this type. The sufficiently free access of sound vibrations to both sides a the plane of the dia of the diaphragm afforded by the arrangement shown causes or permits, for the reasons heretofore considered, the desired equalization of vibration effects upon the two sides for the urpose of eliminating, substantially or who y, sounds from a source relatively distant from the diaphragm or which is in ragm, the diaphragm being, however, e ectively responsive to sound vibrations from a relatively near source and advancing transversely thereto.

A modified transmitter of the carbon-button type is shown in Figs. 15-and 16. This structure is an exemplification of one arrangement of the diaphragm and its casing to permit very complete balancing of the diaphragm and to avoid interference withthe diaphragm action which ma be caused in other arrangements by mun reflections and sound shadows, due to proximity of other parts of the instrument.

The casing 50 encloses in a chamber 51 a variable resistance device 52 of the carbonbutton type. This resistance unit 52 is of similar construction to that of Figs. 10 to 12, except that the cap 31 serves merely to retain diaphragm 20 and other parts of the resistance unit in assembly, and is not formed to enclose the outer end of the button 19. A sleeve 53 is connected to casing 50 and extends from it a considerable distance. The outer end of this sleeve is flared out to provide the rear member 54 of a housing for the main diaphragm D. The diaphragm housing is completed by another member 55 having an internal screw thread 56 engaging the threaded periphery of member 54. The diaphragm is held between the members 54 and 55 in a suitable insulator 56. A stem 57 is firmly secured to the center of the diaphragm D and passes through the central bore of sleeve 53. The diameter of this bore is only slightly larger than the diameter of stem 57, so that sound vibrations are substantially entirely excluded. from the interior of casing 50, and at the same time the longitudinal vibratory movement of stem 57 is not impeded. At its rear end, stem 57 has a screw thread 58 entering a threaded socket in the outward end of button 19. After the diaphragm is put in position with the stem 57 extending through the bore of sleeve 53, the carbon-button unit 52 is placed in chamber 51 and the button is screwed upon the rear end of stem 57 by rotating the unit, a screwdriver slot 59 being provided in the rear face of the button unit for that purpose. After adjustment, the unit 52 is locked against rotation or end movement by a set screw 60 passing through the casin 50, and the rear end of the casing is close by a cap 59.

This transmitter may be conveniently sup rted, as, for instance, on a telephone desk standard of ordinary construction, the upper end of which is indicated in dotted lines at 65, by providing the casing 50 with an ear 66, adapted to fit in the clevis usually provided on such a standard, and having :(Rglt hole 67 to receive the usual clamping The housi members 54 and 55 opposite both faces 0 the diaphragm D, are pro vided with apertures 61 of suitable area and number, and as is readily understood from Fig. 15, the arrangement of parts is such that these apertures may be made'of substantially the same efi'ective area at both sides of the diaphragm, so that equalization of air pressure variations on the two sides of the diaphragm may be made as complete as is desirable or the purposes hereinbefore indicated. There is no large body near the rear of the diaphragm to interfere with the described arrangement of apertures in the diaphragm casing, the cross-sectional area of the stem 53 bein practicall negligible in this respect, and t e length 0 the hollow stem is such that the dia hragm is removed far enough from the car reflect sound vibrations sufliciently to interfere with the desired equalizationof the main diaphragm to any appreciable extent.

What is claimed is:

1. In a telephone transmitter, the combination of a casing havin a tubular extension of small diameter, a iaphragm housing at the end of said extension, a diaphragm therein, a variable-resistance device in said casing, and ,a stem located movably and fitting closely in the bore of said extension and connecting said diaphragm with said resistance device, said dlaphragm housing having apertures affording free and substantially equal access of air-pressure variations to both sides of the diaphragm.

2. In a telephone transmitter, the combination of a support, a casing hin ed thereto, a carbon resistance device in t e casing, a tubular extension on the casing, a relatively narrow housing on the outer end of the extension, a diaphragm in the housing and a connection between the diaphragm and the resistance element extending through the tubular extension, said housing being apern-button housing 50 so that the housing does not impede ortured at both sides to aflord substantially 

